SynonymsCollection -- value_to_group_id_func does not support expansion by default.

[nicolay-r]

It might be better to refactor a default provided by using a separate set of possible grouping providers.

In ARElight, let's say, there is a workaround for a case when expansion is necessary.
https://github.com/nicolay-r/ARElight/blob/702442d162914957b20a4c5804a24714568f9816/arelight/pipelines/serialize_bert.py#L78

The same happen once we declaring a new source, here for example:

AREkit/arekit/contrib/source/rusentrel/entities.py

Line 25 in 040d6b3

value_to_group_id_func=synonyms.get_synonym_group_index),

From the above, it is unclear whether it is allowed an expansion or not.
Hence we may move all of the related implementations into separated file.